Is it possible to unwrap materials made in Vertex Paint?
You can bake Vertex colors to a texure.

Starting from a vertex color layer as in the picture below



you must unwrap your model (which may consist of one or multiple meshes) and create a new image to bake onto ("untitled" in my case)



Than go in the Bake panel of the Render tab and set Bake mode to Vertex color.



When the process is completed you should see something like the picture above.